1. Field of the Invention
The present invention generally relates to a tool holder for attaching a cutting tool to a spindle of a machine tool.
2. Description of Related Art
In machine tools, a cutting tool is exposed continuously to cutting resistance forces and so forth during a cutting process for a work piece, so that vibratory forces are unavoidably generated. Since vibrations generated in the cutting tool have a harmful influence on working precision in machine tools, so that various measures are conventionally taken.
In general, the cutting tools and tool holders are made of materials having high vibration damping characteristics to inhibit vibrations during cutting processes, and/or tool holders are integrally formed with damping materials by welding or shrinkage fit to inhibit vibrations.
In addition to the enhancement of the vibration damping characteristics of the tool holder itself, it is a well-known art to incorporate a damping mechanism in a spindle unit. For example, Japanese Patent Laid-Open No. 1994-277806 discloses a damping mechanism in which elastic body and a viscous fluid are filled between a spindle unit and a spindle head to damp vibrations by the elastic deformation of the elastic body and the viscous resistance of the fluid.
However, it is not possible to expect effective damping capacity by depending on the vibration damping characteristics of the materials themselves of the tools and tool holders like conventional measures. In particular, vibrations are transferred to a machined surface in finishing machining, so that the working precision on the machined surface deteriorates. In addition, the damping materials have a problem in production costs since the damping materials are generally expensive. Moreover, there is a problem in that it is not obtain a damping action effective in the prevention of fretting which is caused on the inner peripheral surface of a tapered hole of a spindle contacting a taper shank of the tool holder.